Accurate determination of geological history in a region or sub-region of a hydrocarbon-bearing province is important in order to build basin-scale, play-scale, and reservoir-scale models of potential hydrocarbon targets. Predictions of oil and gas yields as well as reservoir porosity and permeability are important for assessing hydrocarbon resources. These quantities may be dependent on the thermal and chemical history of a reservoir over geological time. For example, the quality, quantity, and timing of the hydrocarbon may be influenced by the pressures and temperatures to which the source rocks, components thereof, migration pathways, and reservoirs have been subjected. Consequently, models for these properties are important tools for locating and harvesting hydrocarbon reservoirs.
Further, the isotopic signatures of the materials that make up reservoir rocks (such as the minerals, fluids, fossils, and hydrocarbons) reflect their respective geologic, chemical, and biological histories. Such information can be of great relevance to petroleum exploration, production, and development. More specifically, as the rocks are altered in the reservoir, the minerals making up the rocks can attain various isotopic signatures that are uniquely tied to the conditions at the time of the alteration. The resulting signatures may be “frozen” into the sample as the minerals solidify. As the grains age over geologic timescales, the original solidified minerals can be covered by overlayers with other isotopic signatures, indicative of conditions different from those already “frozen” into the interior portions of the grain.
For example, U.S. Pat. No. 4,517,461 to Crandall discloses a carbon isotope analysis of hydrocarbons. The method involves the introduction of a sample containing an isotope of interest into an analytical detector operative to convert the sample into a product analyzable by a mass spectrometer. A property of the sample representative of the quantity of at least one of its constituents is detected, and the conversion product is passed from the detector to a mass spectrometer operative to detect the isotopic ratio of interest. The method optionally includes passing the sample or a source material through a chromatograph column for separation into suitable fractions. The apparatus includes an appropriate analytical detector and a mass spectrometer with optional chromatography column. The method and instrument are particularly suited for analysis of oil-related samples such as crude oil fractions, natural gas, soil gas and oil shale as a tool in oil prospecting.
A number of techniques have been used to analyze the isotope ratios of reservoir samples. These techniques have included, for example, conventional rock analysis, microdrilling or micromilling analysis, laser ablation techniques, and secondary ion mass spectrometry.
In conventional rock analysis, a significant quantity (for example, milligrams to tens of milligrams) of a material is separated from a rock. The separated sample is presumed to be a single type of material, for example, having a relatively uniform matrix. The separated sample is then chemically processed to produce a material for introduction into a mass spectrometer, for example, being dissolved in an acid. The chemically processed sample may be aspirated into a stream of gas that is introduced into the instrument. The chemically processed sample may be ionized during introduction, for example, by being converted to a plasma in this introduction step. Highly accurate values of the isotope ratios may then be determined for the sample.
For example, U.S. Pat. No. 5,012,052 to Hayes discloses an apparatus and method for isotope-ratio-monitoring by gas chromatography-mass spectrometry. With the apparatus and method, samples are introduced in a hydrogen carrier gas into a gas chromatograph and resolved into discrete compounds. The discrete compounds are thereafter introduced to a selectively permeable membrane separator, employing palladium, palladium alloy or other suitable material, to separate out the hydrogen carrier. A replacement carrier gas is simultaneously introduced to carry the chromatographic sample to a combustion reactor, water separator and isotope-ratio-monitoring mass spectrometer. The replacement carrier gas is introduced at a lower flowrate than the hydrogen carrier gas, thus permitting lower flowrates to be introduced to the mass spectrometer to improve its precision. Flowrates to the mass spectrometer are thus reduced without any loss or fractionation of the sample. An improved combustion system is employed to reduce system volume and equalize system pressure, while still providing quantitative combustion.
The traditional use of this method has demonstrated the utility of this information to determine the geologic history of bulk samples. However, due to the quantity of the material used in the analysis, contamination with unwanted materials, for example, materials having other matrices, may be a problem. Separation of the materials prior to chemical processing can be time-consuming and therefore costly. Further, the chemical processing of materials prior to introduction into the mass spectrometer is time consuming, requiring highly specialized analytical chemical skills. See Sharp, Z., PRINCIPLES OF STABLE ISOTOPE GEOCHEMISTRY (Pearson Prentice Hall, Upper Saddle River, N.J., 2007).
In micromilling (or microdrilling) analysis, a small quantity of powder is produced by mechanical abrasion of a target region in a sample, providing spatial resolution and reducing potential contamination versus bulk analysis. The resulting analyzed volume may be cylindrical or trench shaped, depending on the tool used. The minimum volume of material that can be sampled using this technique is approximately 108 μm3, which may be equivalent to a fraction of a milligram of material. The size of the micromill or microdrill tool determines the minimum step size resolution, with values as small as 100 μm reported.
However, rock components of interest may be smaller than the minimum spatial resolution for this method. See Vincent, B., L. Emmanuel, P. Houel, and J. Loreau, Geodynamic Control on Carbonate Diagenesis: Petrographic and Isotopic Investigation of the Upper Jurassic Formations of the Paris Basin (France), 197 SEDIMENTARY GEOLOGY 267-289 (2007). Further, chemical processing of the resulting powder requires time consuming specialized analytical chemical skills, due to the small sample size. As a result, microdrilling or micromilling may be used more often on carbonate-based rock components than on materials having other matrices, as carbonate matrices are softer and more easily dissolved than many other matrices. See C. Spotl and D. Mattey, Stable Isotope Microsampling of Speleothems for Palaeoenvironmental Studies: A Comparison of Microdrill, Micromill and Laser Ablation Techniques, 235 CHEMICAL GEOLOGY 48-58 (2006) (hereinafter “Spotl”).
In laser spectrometry, a focused laser beam can provide spatial analysis with typical spot sizes of about 100 μm. Three methods may be used to sample the material: laser ablation, thermal vaporization, and chemical reaction. In laser ablation, material can be directly removed from the surface of the sample and ionized by the energy from the photons in the laser. The ionized material may be captured by a radio wave generated plasma and released into a induction stream into a mass spectrometer, for example, in techniques such as laser ablation inductively coupled plasma mass spectrometry (laser ablation-ICP-MS). In another technique, the laser may be used as a direct source of heat energy to the surface of the sample, causing vaporization of the sample at a target point. The vaporized material may then be captured in a neutral gas induction stream and fed into a mass spectrometer. In chemical reaction techniques, the laser may energize a reactive gas at the surface of the sample, causing the formation of a corrosive ionized atmosphere in a target area.
However, laser based methods generally require specialized lasers, with frequencies tailored to either the characteristics of the solid sample or to the solid's reaction with a reactive gas environment. Therefore, to analyze a full suite of rock components can require several specialized instruments. Further, a correction to account for mass fractionation during sample generation, specific to that isotope and solid, must be applied. The spatial resolution is significantly larger than the beam size due to beam damage, which is 2-to-4 times the diameter of the beam. See Spotl; C. I. Macaulay, A. E. Fallick, R. S. Haszeldine, and C. M. Graham, Methods of Laser-Based Stable Isotope Measurement Applied to Diagenetic Cements and Hydrocarbon Reservoir Quality, 35 CLAY MINERALS 313-322 (2000).
In secondary ion mass spectrometry (SIMS), a focused ion beam sputters material from a surface of a sample, ejecting ionized particles, which are then introduced into an attached mass spectrometer for analysis. Spot sizes for typical isotopic analysis range in size from several tens of μm in diameter down to 3×3 μm2. The technique is efficient, requiring less than 102 μm3 of material, corresponding to a few nanograms. Although any solid component may be analyzed, variations in the matrices will be reflected in different measured isotope ratios. This is a result of the slight differences in reactivity exhibited by different isotopes of an element. Thus, a matrix correction may be required to provide accurate results. There are several empirical schemes to calculate matrix corrections, but these may not be sufficiently accurate for quantitative geochemistry.
Application of SIMS analysis to rock analysis has been tested by a few groups. For example, one study performed SIMS based on a single matrix correction that did not take into account the effects of variation of compositional changes. See I. R. Fletcher, M. R. Kilburn, and B. Rasmussen, 2008, Nanosims Mu μ-Scale In Situ Measurement of C-13/C-12 in Early Precambrian Organic Matter, with Permil Precision, 278 INTERNATIONAL JOURNAL OF MASS SPECTROMETRY 59-68. Although the technique has a high spatial resolution, the full advantages of the theoretical limits have not been realized, and as a consequence, thin microquartz rims were missed in some analyses of oxygen isotopes. See A. M. E. Marchand, C. I. Macaulay, R. S. Haszeldine, and A. E. Fallick, Pore Water Evolution in Oilfield Sandstones: Constraints from Oxygen Isotope Microanalyses of Quartz Cement, 191 CHEMICAL GEOLOGY 285-304 (2002); N. E. Aase, and O. Walderhaug, The Effect of Hydrocarbons on Quartz Cementation: Diagenesis in the Upper Jurassic Sandstones of the Miller Field, North Sea, Revisited, 11 PETROLEUM GEOSCIENCE 215-223 (2005).
Although the use of microanalytic techniques is widespread, these techniques are seldom applied to problems associated with the reservoir rocks of hydrocarbon reservoirs. The predominant applications of the prior microanalytical techniques to natural samples are in characterizing mantle geochemistry, cosmochemistry, paleo-oceanography, and age dating. Isotopic and trace element chemistry is often used for mantle geochemistry. The use of microanalytics for cosmochemistry is driven by the fact that sample sizes are nearly always limited and the geochemical questions are closely related to mantle geochemistry. The interest of the paleo-oceanographers is driven by interest in global climate change.